MANILA (Reuters) – Chinese coast guard vessels prevented a Philippine nationalist group from planting a Filipino flag on a rocky South China Sea outcrop, the group said on Monday, the latest territorial standoff between the two countries.

This photo posted on Sunday by Kalayaan Atin Ito (Kalayaan This Is Ours) shows Filipino activists posing for photo at the bow of their vessel next to a Philippine flag at Scarborough Shoal

The incident between the coast guard and the Kalayaan Atin Ito (Freedom It’s Ours) group took place at the disputed Scarborough Shoal on Sunday, just as foreign ministers from Southeast Asian countries and China prepared for a meeting in Kunming to discuss territorial rows in the hotly contested waters.

Beijing claims almost the entire South China Sea and the move by the Filipino nationalists comes as the Permanent Court of Arbitration in the Hague prepares to deliver a ruling on a complex case brought by Manila that could dent China’s sweeping sovereignty claim.

“They refused to allow us to get near Scarborough Shoal,” Joy Ban-eg, leader of the Kalayaan Atin group, told reporters. “There was a standoff until we decided to leave.”

Ban-eg said 15 Filipinos and an American joined the 16-hour voyage to the Scarborough Shoal to mark the Philippines’ 118th Independence Day and to find out if local fishermen could freely go there.

The shoal, seized by China after a three-month standoff in 2012, is a bone of contention for the Philippines and its president-elect, Rodrigo Duterte, has vowed not to give way over the right of his country to sail there freely.

The attempt to plant the flag comes after Duterte himself pledged during his election campaign to do the same, but on China’s man-made islands in the Spratlys, using a jet ski.

The wooden-hulled fishing boat came close to the shoal when China’s coastguard blocked them and ordered them to go back to the Philippines, the group said.

Five Filipinos jumped on the water and tried to swim to the shoal but were chased down by Chinese sailors on rubber dinghies who sprayed them with water and tried to take their cameras and bag, which contained a Philippine flag, they said.

Philippine defense and military officials declined to comment on the incident. Beijing stressed that the shoal belonged to China.

“The Scarborough Shoal has been China’s territory since ancient times,” Chinese Foreign Ministry spokesman Lu Kang said at a regular briefing in Beijing.